Filtering apparatus

ABSTRACT

A filtering apparatus for a reflow oven includes a contaminated-gas inlet for receiving contaminated gas from a reflow oven; a filtering device configured to filter the received contaminated gas; a returned gas outlet for returning filtered gas to the reflow oven; a gas removal arrangement for allowing gas to escape from the apparatus; and a control arrangement for controlling the rate of gas flow through the gas removal arrangement such that in use the flow rate of contaminated gas flowing from a hotter region of the reflow oven to a cooler region of the reflow oven is controlled.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S.C. §119(a)-(d)of British application serial no. GB 0113043.4 filed May 30, 2001, andunder 35 U.S.C. §120 of prior U.S. application Ser. No. 09/907,103 filedJul. 17, 2001, the disclosures of which are incorporated by referenceherein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to filtering apparatus for a reflowoven and a system for reflow soldering.

[0004] 2. Description of the Related Art

[0005] Reflow soldering is now an established method of solderingelectronic components to printed circuit boards. The printed circuitboards are produced using a screen printing process in which solderpaste containing solder, flux, adhesives and binders is applied to aboard in a required configuration. Components are then glued to theprinted circuit board at appropriate locations by a pick and placemachine. The components may be applied to just one side of the board or,in other cases, both sides.

[0006] The reflow soldering process typically takes place by passing aprinted circuit board, with its components attached, along the conveyorbelt through the tube (or tunnel) of a reflow oven. Within the oven, theboard passes through a high temperature region, typically in excess of200 degrees centigrade, in which the solder melts and forms a jointbetween the circuit of the board and the respective components. Fluxwithin the paste reacts with metallic surfaces to remove oxide andenhance wetting. The soldering process takes place at a hightemperature, but if the temperature is too high damage is caused to, atleast, the more sensitive electronic components. Thus, the temperaturemust be carefully controlled within pre-defined limits, the limitsthemselves depending on the particular components, solder paste, etc.that are being used.

[0007] After the solder joints are formed, the conveyor passes the boardthrough a cooling region of the oven, in which the solder solidifies,before it emerges from the oven.

[0008] The oven typically contains a process gas atmosphere of nitrogen,so that oxidation of the board and components is minimized duringheating. Consequently, fresh nitrogen is continually input into the ovenat considerable expense.

[0009] Heat within the oven vaporizes unreacted flux, binders andadhesives contained within the solder paste, while other vapors areliberated by the reaction of the flux on the oxidized contacts ofcomponents. Thus, the nitrogen atmosphere becomes contaminated by theaforesaid vapors.

[0010] If the contaminated nitrogen migrates into the cooling region orcooler regions of the oven, at least some of the vapors will condense onthe cooler surfaces and may drip onto the circuit boards, thus producingdefects. Consequently, it is known to have filtering apparatus whichextracts contaminated process gas from the oven, filters it to removemost of the flux vapors, and re-inputs the filtered gas into the oven.However, contaminated gas is replaced by filtered gas, new cool nitrogenis continually input to the oven, and the new cool nitrogen expands onheating; therefore the net result is that contaminated process gas tendsto be pushed out from the ends of the oven's tunnel. Since thecontaminated gas must pass through cooler regions of the oven on its wayto the ends of the tunnel, the aforementioned condensation occurs.

[0011] In addition, in known systems, the contaminated gas escaping fromthe ends of the tunnel is sucked away to be expelled to the open air,possibly via a filtration unit. The expelled gas is typically passedthrough ducting out into the air above the roof of the working area.Such a system has several disadvantages as a consequence. Firstly, sincethe ducting exhausts the gas above the roof, the variability of windspeed can cause variations in the oven's operation, in particular, itsworking temperatures. Secondly, if for any reason the oven is requiredto be relocated, the cost of such is increased due to the need toprovide ducting through the roof. Thirdly, providing suction at the endsof the oven's tunnel tends to increase the flow of contaminated gasalong the tunnel and enhance the previously discussed condensationproblem.

BRIEF SUMMARY OF THE INVENTION

[0012] According to a first aspect of the present invention, there isprovided a filtering apparatus for a reflow oven, said apparatuscomprising: a contaminated-gas inlet for receiving contaminated gas froma reflow oven; filtering means configured to filter said receivedcontaminated gas; a returned gas outlet for returning filtered gas tosaid reflow oven; a gas removal means for allowing gas to escape fromsaid apparatus; and a control means for controlling the rate of gas flowthrough said gas removal means such that in use the flow rate ofcontaminated gas flowing from a hotter region of said reflow oven to acooler region of said reflow oven is controlled.

[0013] According to a second aspect of the present invention, there isprovided a system for reflow soldering comprising: a reflow oven;filtering means configured to filter contaminated gas received from saidreflow oven; a means for returning filtered gas to said reflow oven; agas removal means for allowing gas to escape from said system; and acontrol means for controlling the rate of gas flow through said gasremoval means such that in use the flow rate of contaminated gas flowingfrom a hotter region of said reflow oven to a cooler region of saidreflow oven is controlled.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014]FIG. 1 shows a reflow soldering system 101;

[0015]FIG. 2 shows schematically components within the reflow oven 102,along with the filtering unit 103;

[0016]FIG. 3 shows a view along the conveyor belt 105 of the heatingunits 211 and 231;

[0017]FIG. 4 shows schematically, selected components within the reflowoven 102, along with the second filtering unit 104;

[0018]FIG. 5 shows a perspective view of a push-pull extraction unit421;

[0019]FIG. 6 shows a plan view of the push-pull extraction unit 421;

[0020]FIG. 7 shows schematically, selected components of the oven 102,along with the filtering units 103 and 104;

[0021]FIG. 8 shows a device 801 for measuring gas flow-at the ends ofthe tunnel 270;

[0022]FIG. 9 shows an alternative filtering unit 901 of an alternativesystem.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023]FIG. 1

[0024] A reflow soldering system 101 is shown in FIG. 1. The system 101comprises a reflow soldering oven 102, and filtering units 103 and 104.

[0025] Prior to reflow processing, printed circuit boards are producedby a screen printing process, in which solder paste is applied to aboard, and then electronic components are attached by a pick and placeprocess. The printed circuit boards with attached components are thenapplied to a conveyor belt 105 at a first end 106, referred to as the onload end, of the reflow oven 102.

[0026] The conveyor belt 105 has a first exposed portion at the on loadend 106 and a second exposed portion at the opposite end 107 of theoven, referred to as the off load end. Between the two exposed portions,the belt is enclosed by a tube, referred to as a tunnel within the oven.The oven is configured such that it can maintain a particulartemperature profile along the tunnel. Thus, an unsoldered printedcircuit board applied to the belt 105 at the on load end 106, istransported through hot and cooler regions of the oven's tunnel andemerges with soldered joints via an opening 108 at the off load end. Inaddition, the oven is supplied with nitrogen, such that during use thetunnel contains a nominal atmosphere of nitrogen. By providing anon-oxidizing process gas atmosphere, in the form of nitrogen, the boardand its components are saved from oxidation during the heating process.

[0027] During the passage of printed circuit boards through the tunnel,the board out-gases, flux within the solder paste reacts with themetallic contacts of the electronic components to produce vapors, otherunreacted portions of the flux evaporate and other constituents of thesolder paste also evaporate.

[0028] The first filtering unit 103 is primarily concerned withmaintaining the flux vapor levels within the oven below tolerablelimits, while the second filtering unit 104 manages the rate of gasexhausted from the system and cleans said gas such that it can be safelyexhausted within the workspace of the oven which is occupied by humanoperators. Furthermore, the filtering unit is configured to cool theexhausted gas, so that it does not heat the surrounding air tointolerably high temperatures for the human operators. Although thefunction of the two filtering units can be clearly distinguished in sucha way, it should be understood that the two units interact to maintainthe efficient operation of the system, as will be described later.

[0029]FIG. 2

[0030] Components within the reflow oven 102 are shown schematicallywithin dashed line 201 in FIG. 2, along with the filtering unit 103. Theupper surface of the tunnel 270 is defined by heating units 211 to 223positioned above it and the lower surface of the tunnel is defined byheating units 231 to 243 positioned below. The heating units arearranged in pairs with one unit of a pair positioned directly above theother, such that any part of the tunnel is heated by one pair of heatingunits, e.g. units 211 and 231. The portion of the conveyor belt 105within the tunnel moves from left to right as viewed in FIG. 2, and soit is able to transport printed circuit boards through the pairs ofunits, from units 211 and 231 to the final pair of units 223 and 243.

[0031] Each heating unit has a gas outlet connected by its outlet branchpipe and a main pipe 208 to an inlet 203 of filtering unit 103. Thecorresponding branch pipe of each heating unit includes a solenoidactivated valve which prevents gas flow when closed. For example,heating unit 211 has an outlet 205 connected by branch pipe 207 and mainpipe 208 to inlet 203, and the branch pipe 207 includes a solenoidactivated valve 209. In a similar manner, each heating unit has a gasinlet connected by an inlet branch pipe and a main pipe 250 to an outlet204 of filtering unit 103. The corresponding inlet branch pipe of eachheating unit includes a solenoid activated valve which prevents gas flowwhen closed. For example, heating unit 211 has an inlet 251 connected byinlet branch pipe 252 and main pipe 250 to outlet 204, and the inletbranch pipe 252 includes a solenoid activated valve 253.

[0032] The inlet 203 connects main pipe 208 to a first chamber 254 offiltering unit 103. Chamber 254 contains a heat exchanger 255 cooled bychilled gas received via pipe 256 and returned via pipe 276. As will bedescribed later, the chilled gas is generated by the second filteringunit 104. The first chamber 254 is connected to a second chamber 257 ofunit 103 via a pipe 258 having an in-line Coanda gas mixer 259. The gasmixer 259 also receives fresh nitrogen gas from a liquid nitrogencylinder via a pipe 260. The pipe 258 is connected to the inlet of afilter-bag 261 located within second chamber 257.

[0033] The filter-bag 261 comprises eight connected envelopes offiltering material defining a corresponding eight connectedcompartments. Due to this configuration, the filter-bag has a largesurface area providing filtering.

[0034] The second chamber 257 has an outlet 262 connecting it to a thirdchamber 263. Chamber 263 contains a centrifugal fan 264 located adjacentto the outlet 262 such that in operation it is able to draw air inthrough outlet 262. The outlet 204 of unit 103 connects the thirdchamber 263 with main pipe 250.

[0035] Branching from main pipe 208 is a gas removal pipe 265 which hasan inline balancing valve 266. The valve 266 is configured to beadjustable to allow a variable rate of gas flow from main pipe 208, andin operation it is adjusted in response to tunnel gas flow ratemeasurements, as described later. The gas removal pipe 265 connects tothe inlet of the second filtering unit 104, and this will also befurther described later.

[0036] In operation each pair of heating units supplies heat to thetunnel, such that a corresponding region of the tunnel is maintained ata specified temperature. By this means, in a typical reflow process, theboards pass through several regions of increasing temperatures up to aparticular high temperature (typically between 200 and 300 degreescentigrade depending on the solder paste being used). They then passthrough several regions all held at that particular high temperaturewhile the solder joints are formed. The boards then pass through severalregions having successively lower temperatures and are therefore cooledbefore exiting the tunnel. Therefore, typically the temperature of thetunnel between units 211 and 231 is relatively cool, while the region ofthe tunnel corresponding to units 212, 213, 214 is successively hotter.A region of the tunnel corresponding to a number of units, e.g. units217, 218, 219 and 220, is at a particular high temperature, providingthe “soak area” in which the solder wets the components and forms thejoints. The regions corresponding to the following remaining units, e.g.units 221, 222 and 223, are then successively cooler. As a result, thetemperature of the tunnel at its ends is low compared to an intermediateregion.

[0037] During the reflow process, as a printed circuit board passesalong the tunnel, from region to relatively hotter region, itstemperature gradually increases. As a consequence, the variousconstituents of the flux are evaporated depending upon their respectivevapor pressures at the temperatures concerned. In addition, other vaporsare produced as the flux reacts with the contacts of the components ofthe board. The nitrogen atmosphere within the tunnel thus becomescontaminated by flux vapors.

[0038] In order to manage the levels of flux vapors within the tunnel,contaminated gas is extracted from particular ones of the heating unitsvia pipe 208, filtered by unit 103 and the filtered gas is returned tothe same particular units via pipe 250. The choice of which units havetheir gas extracted is determined from a knowledge of the temperaturesof each of the units and the volatilization rates of the constituents ofthe flux. The necessary information regarding volatilization rates ismade available by solder paste manufacturers as a TGA diagram.Specifically, the units which have temperatures corresponding to highvolatilization rates are identified as those requiring extraction andthe corresponding solenoid valves are therefore opened.

[0039] In addition, when the contaminated gas is extracted fromparticularly hot heating units, solenoid valves corresponding to one ormore of the cooler heating units in the warm up sequence, such as units211, 231, are opened so that the gas arriving at the filtering unit isalready cooled to some degree. Thus a first stage of cooling is providedeven before the hot contaminated gas arrives at the filtering unit 103.

[0040] In operation, nitrogen gas contaminated with flux vapors is drawnfrom the selected heating units and the larger part of this entersfiltering unit 103, where it is cooled by heat exchanger 255. The cooledcontaminated gas is then mixed with fresh nitrogen gas within the Coandagas mixer 259. Since the fresh nitrogen gas is relatively cool, being ator below ambient temperature, it cools the contaminated gas furtherbefore the mixture of gases enters the filter bag 261. The three stagesof cooling are each adjustable to ensure that the final temperature ofthe mixed gases is sufficiently low to cause the flux vapors to condenseand also maintain the filter bag temperature at required levels.Furthermore, by controlling the temperature in this way, conventionalfiltering media, such as non-woven polyester, may be used in theconstruction of the filter-bag.

[0041] It should also be noted that by using nitrogen in the filteringunit 103 to assist cooling of contaminated gas, at the same time, thecontaminated gas is used to heat up the fresh nitrogen. Thus energy maybe saved in comparison to existing systems, in which fresh nitrogen ispiped directly into the oven.

[0042] The filtered gas which escapes from the filter-bag is then drawninto chamber 263 by the centrifugal fan 264. The increased pressure inchamber 263 forces the filtered gas back to the heating unit via pipe250. It should be noted that the movement of gas to and from the filterunit 103 is primarily caused by the centrifugal fan 264, but is alsoassisted by fans located in the heating units such as 211.

[0043]FIG. 3

[0044] A view along the conveyor belt 105 of the heating units 211 and231 is provided in FIG. 3. The heating units such as 211 and 231comprise of a plenum chamber 301 containing a centrifugal fan 302located adjacent to an inlet 303, such that in operation the fan 302draws air into the plenum chamber 301. The plenum chamber 301 has anelectrically heated perforated wall 304 which forms the ceiling of thetunnel in the case of upper units such as 211, and forms the floor ofthe tunnel in the case of the lower units such as 231. The plenumchamber 301 is located within an outer case 305 such that a passageway306 is formed between the side and upper walls of the plenum and thecorresponding walls of the case. The passageway has openings 307adjacent to the perforated wall 304 and it is also in communication withplenum 301 via plenum inlet 303.

[0045] The outlet 205 of the heating unit 211 provides communicationbetween the plenum chamber and the outlet branch pipe 207, and similarlyinlet 250 provides communication between passageway 306 and inlet branchpipe 251.

[0046] During operation the high pressure within the plenum 301, causedby fan 302, forces air through the perforations in the heated wall 304.Thus, heated nitrogen gas is blown onto the conveyor belt 105 and, ofcourse, onto any printed circuit boards, such as printed circuit board308, presently being transported by the conveyor in the correspondingregion of the tunnel. The gas is then drawn through opening 307 andpassageway 306 by the fan 302, and then back into the plenum 301. Whenthe corresponding solenoid valves are open, gas will also be forced outof plenum 301 into outlet branch pipe 207, to be filtered, while a flowof filtered gas is drawn into passageway 306 from inlet branch pipe 252.It may be noted that the pressure differentials produced by the fan 302assist the circulation of gas to and from filtering unit 103.

[0047] In an alternative embodiment, contaminated gas is drawn from thepassageway 306 for filtering and the filtered gas is then returned tothe plenum 301. This system operates against the pressure differentialproduced by the centrifugal fan 302. However, it has the advantage thatthe gas in the passageway which is relatively dirty is drawn away forfiltering, whereas the plenum chamber gas is relatively clean, since itpartly comprises of the newly received filtered gas and the freshnitrogen.

[0048]FIG. 4

[0049] Selected components within the reflow oven 102 are shownschematically within dashed line 401 in FIG. 4, along with the secondfiltering unit 104. The gas removal pipe 265 provides communicationbetween the balancing valve 266 and the inlet 402 of a filter-bag 403located within a first chamber 404 of the filtering unit 104. Thefilter-bag 403 is of substantially the same type as filter-bag 261. Aconnecting pipe 405 connects the first chamber 404 with the lower partof a second chamber 406 containing a HEPA (high efficiency particlearrestor) and chemical filter 407, and a heat exchanger 408. Thestandard of filtration of filtering unit 104 is therefore sufficientlyhigh so as to produce gas which may be released into the workspace. Theheat exchanger 408 is cooled by chilled water received from, andreturned to, a reservoir (not shown) via pipes 409. The second chamber406 has an outlet 410 connecting it with a third chamber 411. Thechamber 411 contains a centrifugal fan 412 located adjacent to theoutlet 410, such that it draws gas through outlet 410 from chamber 406and into chamber 411.

[0050] The third chamber 411 of filtering unit 104 has outlets 413 and414 connecting it with gas jet supply pipes 415 and 416 respectively.The pipe 256 which supplies cooled gas to the heat exchanger 255 offiltering unit 103 branches from gas jet supply pipe 415. The gas isreturned to the filtering unit 104 from the heat exchanger 255 via pipe276 connected to gas removal pipe 265.

[0051] A system outlet pipe 417 branches from gas jet supply pipe 416allowing gas to be released from the system 101 via room air bleed valve440.

[0052] The gas jet supply pipes 415 and 416 connect with one of two gasjets 418 and 419 respectively. Each of the gas jets 418 and 419 formpart of a push-pull extraction unit 420 and 421 respectively, locatedadjacent to each of the open ends of the tunnel. The push-pullextraction units 420 and 421 each have a gas receiving funnel 422 and423 which is connected to gas removal pipe 265 via pipes 424 and 425.

[0053] A pipe 427 connected to gas removal pipe 265, has an in-line roomair inlet valve 426 which is adjustable to allow variable rates of roomair into gas removal pipe 265.

[0054] In operation, hot contaminated gas is received into filter unit104 via balancing valve 266. As mentioned earlier, the rate of flow ofsaid hot contaminated gas is determined by the adjustment of valve 266,but it is also variable by adjusting the speed of the centrifugal fan412. The hot contaminated gas received from valve 266 is mixed withrelatively cool gas received from pipes 424 and 276. On reaching thefilter-bag 403 the mixed gases are arranged to have sufficiently lowtemperature to condense the flux vapours. If it is required to cool thegas further, room air is bled into pipe 265 via room air valve 426. Themixture of gases is therefore pre-filtered by filter-bag 403 beforebeing filtered by the HEPA/chemical filter 407 and cooled by heatexchanger 408. The gas emerging from the filtering unit is sufficientlyclean so as to be releasable into the workspace and because it is coolit does not raise the ambient temperature of the workspace when releasedvia room air bleed valve 418.

[0055] However, only a portion of this cleaned cooled gas is released.As previously described, a portion of said cooled gas is used to coolheat exchanger 255 in filtering unit 103, while other portions aresupplied to gas jets 418 and 419.

[0056] Gas jets 418 and 419 are configured to provide a flow of cooledgas from one side of a tunnel opening, across said opening, to the otherside, towards respective funnels 422 and 423. In addition, due to theaction of centrifugal fan 412, the pressure within pipes 424 and 425 isbelow atmospheric pressure and so the gas flow from the gas jets isdrawn into the receiving funnels and then into the corresponding pipes.

[0057] In practice, a limited degree of contaminated gas escapes fromthe ends of the tunnel but this is entrained by the gas flow from thejets and is therefore sucked into the gas receiving funnels.

[0058] The flow of gas across the ends of the funnels tends to draw gasfrom within the tunnel by the Venturi effect. However, this is smallcompared to the effect of conventional exhaust systems which only suckaway the escaping contaminated gas. Because the present arrangementdraws less gas from the ends of the tunnel when compared to conventionalsystems, less fresh nitrogen needs to be input to the system inreplacement.

[0059] It will now be understood that the gas emerging from pipe 424 andentering pipe 265 is contaminated to a degree and is also warm being amixture of cooled gas and oven gas. Within pipe 265, it is mixed withgas from other sources as previously described such that the temperatureof the final mixture entering filtering unit 104 is sufficiently low toallow efficient filtering by pre-filter 403.

[0060]FIGS. 5 and 6

[0061] Push-pull extraction unit 421 is illustrated in the perspectiveview of FIG. 5, and the plan view of FIG. 6. The extraction unit ismanufactured from stainless steel sheet. However, gas jet supply pipe416, an outside panel to which said pipe is connected, and a top panelhaving an edge 501 are shown in FIG. 5 as being transparent, in order toshow the structure of the extraction unit 421 more clearly.

[0062] The push-pull extraction unit 421 is positioned at the end of thetunnel 270 around conveyor belt 105. As described above, the gas jet 419is located at one side of the end of the tunnel 270 and the gasreceiving funnel 423 is located at the opposite side.

[0063] The gas jet 419 is configured to have a compartment whichreceives gas from pipe 416 and allows gas to exit via a rectangularaperture 503. The receiving funnel 423 has a wide open end adjacent tothe conveyor belt and a narrower end from which extends pipe 425.

[0064] The small volume of contaminated gas which escapes from the endof the tunnel is indicated by arrows 601 in FIG. 6, while the flow ofgas from gas jet 419 to funnel 423 is indicated by arrows 602.

[0065]FIG. 7

[0066] Selected components of the oven 102 are shown schematicallywithin a dashed line 701 in FIG. 7 along with the filtering units 103and 104. The features of FIG. 7 are identified by the same numbers asused in the preceding Figures. Therefore, included within the oven areheating units 211 to 223 and 231 to 243, and main pipe 208 is used totransport contaminated gas from the heating units to the inlet of firstfiltering unit 103, while gas removal pipe 265 connects pipe 208 withthe inlet of filtering unit 104, via balancing valve 266.

[0067] If, during operation, balancing valve 266 were closed, the massof gas within the oven and filtering unit 103 would increase due to thefresh nitrogen input at the Coanda gas mixer 259. This would result ingas being forced to escape from the ends of the tunnel, and consequentlycontaminated gas from hotter regions of the tunnel would have to passthrough the cooler regions at either end of the tunnel. This wouldresult in condensation on the cooler surfaces within the tunnel andproduce the previously discussed problems. This unwanted gas flow isalso exacerbated by the expansion of the fresh nitrogen gas as itstemperature increases. Furthermore, as flux volatilizes in the tunnel,this also causes increased volume of gas in the tunnel, tending to forceexcess gas along the tunnel and out through its ends.

[0068] In order to avoid the unwanted gas flow along the tunnel,balancing valve 266 is opened to allow a proportion of the gas in mainpipe 208 to be drawn into filtering unit 104 via gas removal pipe 265.By allowing gas through valve 266 at the required rate the gas flow fromthe ends of the tunnel is minimized. It should be noted that a zero rateof flow from the ends of the tunnel would be ideal, but a flow into theends of the tunnel is undesirable since the atmosphere in the tunnelwould then become contaminated with atmospheric oxygen. Therefore, inpractice, the gas flow from the ends of the tunnel is kept to a positiveminimal value by adjusting the valve 266 and/or the speed of centrifugalfan 412, in dependence of measurements of said gas flow. Thecondensation of flux vapors on cool surfaces of the oven is thereforenot totally eliminated but is greatly reduced such that it does notcause problems before a scheduled shut down of the oven for cleaningprocedures.

[0069]FIG. 8

[0070] A device 801 for measuring gas flow at the ends of the tunnel 270is shown in FIG. 8. The device 801 is a Pitot tube which provides anelectrical output from a pressure transducer 802 in dependence of thepressure appearing at aperture 804 perpendicular to the gas flow, andaperture 805 which is parallel to the gas flow. As is known, theelectrical output thus provides a value from which the velocity of thegas flow is determined.

[0071] As shown in FIG. 8, the device 801 is located just inside the endof the tunnel towards its ceiling, where gas flow tends to be largest.It is also located between the final heating units 223,243 in the seriesand the push-pull extraction unit 421, so as not to be affected by localgas flows caused by said units.

[0072] The balancing valve 266 and/or the speed of the fan 412 istherefore adjusted in response to the value of the electrical outputfrom the device 801. This is done automatically by the filtering unit104 which is controlled by a microcontroller (not shown).

[0073]FIG. 9

[0074] An alternative filtering unit 901 of an alternative system isshown in FIG. 9. The alternative system is the same as the system 101except that the first filtering unit 103 is replaced by the alternativefiltering unit 901. The alternative filtering unit 901 is alike tofiltering unit 103 and therefore it has a first chamber 254 containing aheat exchanger 255, a connecting pipe 258 from the first chamber to asecond chamber 257, a filter-bag 261 within the second chamber and athird chamber 263 containing a centrifugal fan 264. However, the pipe258 is not connected to a Coanda gas mixer. Instead, a pipe 902 carryingfresh nitrogen gas from a cylinder, is arranged to extend through partof pipe 258, adjacent to the inlet 903 of the filter bag 261. The pipe902 is blanked off at its end nearest to the filter-bag, but the portionof pipe within the pipe 258 has a series of circular apertures 905equally spaced along its length and around its circumference.

[0075] In operation the relatively cool nitrogen escapes from apertures905 and mixes with the contaminated gas passing through pipe 258,thereby cooling the contaminated gas before it is filtered by thefilter-bag. The pipe 902 therefore provides an alternative means ofmixing fresh process gas with contaminated gas.

[0076] In the described embodiments pipes are used as the channellingmeans for transporting gases around the system. However, in alternativeembodiments, some of said pipes are replaced by ducting.

1. A filtering apparatus for a reflow oven, said apparatus comprising: acontaminated-gas inlet for receiving contaminated gas from a reflowoven; a filtering device configured to filter said received contaminatedgas; a returned gas outlet for returning filtered gas to said reflowoven; a gas removal arrangement configured to allow gas to escape fromsaid apparatus; and a control arrangement operative to control the rateof gas flow through said gas removal arrangement in dependence on a flowrate of gas within said reflow oven, whereby the flow rate ofcontaminated gas flowing from a hotter region of said reflow oven to acooler region of said reflow oven is controlled.
 2. A filteringapparatus according to claim 1, wherein the apparatus includes a gaschannelling arrangement for transporting contaminated gas from the ovento the contaminated-gas inlet, and the control arrangement controls therate of gas from the channelling arrangement to the gas removalarrangement.
 3. A filtering apparatus according to claim 1, wherein theapparatus includes a measuring device configured to provide anindication of the flow rate of contaminated gas flowing from the hotterregion of the reflow oven to the cooler region, such that the controlarrangement is adjustable in response to the indication of the flowrate.
 4. A filtering apparatus according to claim 1, wherein theapparatus includes a fresh process gas inlet and a gas mixingarrangement configured to mix fresh process gas with contaminated gasreceived from the oven before the contaminated gas is filtered.
 5. Afiltering apparatus according to claim 1, wherein the fresh process gasis nitrogen.
 6. A filtering apparatus according to claim 1, wherein theapparatus includes: a further filtering device for filtering thecontaminated gas to produce cleaned gas, such that the cleaned gas isreleasable into a user occupied space; a cooling device for cooling thecleaned gas to produce cooled gas; and an outlet configured to exhaustthe cooled gas.
 7. A filtering apparatus according to claim 6, whereinthe gas removal arrangement is configured to allow a flow of gas intothe further filtering device.
 8. A filtering apparatus according toclaim 7, wherein the apparatus is configured for use with a reflow ovenhaving an opening for allowing soldered products to be removed from theoven, and the apparatus further comprises: a blowing device configuredto blow the cooled gas across the opening thereby deflecting gasescaping from the opening; a gas inlet for receiving a mixture of thecooled gas and the gas escaping from the opening; and a filtering devicefor filtering the mixture of gases received by the gas inlet.
 9. Afiltering apparatus according to claim 1, wherein the apparatus isconfigured for use with a reflow oven having an opening for allowingsoldered products to be removed from the oven, and the apparatus furthercomprises: a blowing device configured to blow relatively clean gasacross the opening thereby deflecting gas escaping from the opening; agas inlet for receiving a mixture of the clean gas and the gas escapingfrom the opening; and a filtering device for filtering the mixture ofgases received by the gas inlet.
 10. A system for reflow solderingcomprising: a reflow oven; a filtering device configured to filtercontaminated gas received from the reflow oven; an arrangement forreturning filtered gas to the reflow oven; a gas removal arrangementconfigured to allow gas to escape from the system; and a controlarrangement operative to control the rate of gas flow through the gasremoval arrangement in dependence on a flow rate of gas within thereflow oven, whereby the flow rate of contaminated gas flowing from ahotter region of the reflow oven to a cooler region of the reflow ovenis controlled.
 11. A filtering apparatus for a reflow oven, theapparatus comprising: a contaminated gas inlet for receivingcontaminated gas from a reflow oven; a filtering device configured tofilter the contaminated gas; a gas outlet for returning filtered gas tothe reflow oven; and a fresh process gas inlet arranged to allow freshprocess gas to enter into and pass through the filtering device, wherebythe filtering device is cooled and said fresh process gas is warmedbefore entering said oven.
 12. A filtering apparatus configured to cleangas contaminated by an electronics manufacturing process, said apparatuscomprising: an inlet for receiving contaminated gas from an electronicsmanufacturing process; a first filtering arrangement on a firstcirculation path through the electronics manufacturing process operativeto filter gas to a contamination level below a selected threshold forthe electronics manufacturing process; a second filtering arrangement ona branch path off the first circulation path operative to filter gas toa contamination level below a selected threshold for a user-occupiedspace; a cooling arrangement on the branch path operative to cool saidfiltered gas to produce cooled gas; and an outlet configured to exhaustsaid cooled gas to the user-occupied space.
 13. A filtering apparatusaccording to claim 12, wherein the apparatus is configured as a singleunit.
 14. Gas extraction apparatus for a reflow oven having an openingfor allowing soldered products to be removed from said oven, saidapparatus comprising: a blowing device configured to blow relativelyclean gas across said opening onto a flow path out of the reflow oven,thereby deflecting gas escaping from said opening onto the flow path outof the reflow oven; a gas inlet on the flow path for receiving a mixtureof said clean gas and said gas escaping from said opening; and afiltering device on the flow path and configured to filter said mixtureof gases received by said gas inlet.
 15. A system for reflow solderingcomprising: a reflow oven having a plurality of regions, the regionsproducing flux vapor at various rates, at least one region comprising apre-heat region, at least another region comprising a high-temperatureregion at or above a solder reflow temperature; a plurality ofcontaminated-gas outlets configured to receive contaminated gas fromcorresponding ones of the plurality of regions of the reflow oven; afiltering arrangement configured to filter contaminated gas receivedfrom the reflow oven; a plurality of cleaned-gas inlets configured toreceive cleaned gas from the filtering arrangement and to introducecleaned gas into corresponding ones of the plurality of regions of thereflow oven; and a control arrangement operative to control the rate offlow of gas through each of the plurality of the outlets individuallysuch that contaminated gas is extractable from selected ones of theplurality of regions of the reflow oven.
 16. The system of claim 15,wherein the control arrangement is operative to extract contaminated gasfrom the region producing flux vapors at the highest rate.
 17. Thesystem of claim 15, wherein the control arrangement is operative tocontrol the rate of flow of gas through the filtering arrangement independence on a flow rate of gas within the reflow oven, whereby theflow rate of contaminated gas flowing from a hotter region of the reflowoven to a cooler region of the reflow oven is controlled.
 18. The systemof claim 15, wherein the filtering arrangement is operative at atemperature to condense the flux vapor.
 19. The system of claim 15,wherein said plurality of inlets each include a valving arrangement tocontrol the flow of gas therethrough.
 20. The system of claim 15,wherein said plurality of outlets each include a valving arrangement tocontrol the flow of gas therethrough.
 21. The system of claim 15,wherein the filtering arrangement includes a gas moving device operativeto move gas through the filtering arrangement.
 22. The system of claim21, wherein the gas moving device comprises a fan.
 23. The system ofclaim 15, wherein the filtering arrangement further comprises a freshprocess gas inlet arranged to allow fresh process gas to enter into andpass through the filtering arrangement, whereby the filteringarrangement is cooled and the fresh process gas is warmed beforeentering the oven.